Method and apparatus for determining a magnification factor a radiographic image

ABSTRACT

A method and apparatus for determining a magnification factor in a radiography device of the type comprising an X-ray source and means for acquiring images placed facing the source, the source and the means for acquiring images being mounted so as to rotate about at least one axis with respect to a support on which an object to be X-rayed is intended to be positioned. The method and the apparatus implementing the method comprises: acquiring at least two images corresponding to two different angular positions of the source and of the recording means with respect to the support; identifying on these images projections of at least one point of the X-rayed object; and determining the magnification factor of at least one of the images, first, as a function of the angular displacement of the source and of the recording means between the acquisitions of the images in question and, secondly, as a function of the positions on these images of the identified projections.

BACKGROUND OF THE INVENTION

[0001] This application claims the benefit of a priority under 35 USC119 to French Patent Application No. 01 04 160 filed Mar. 28, 2001, theentire contents of which are incorporated by reference.

[0002] The invention relates to image acquisition methods by an X-rayradiography apparatus designed, in particular, for angiography.

[0003] In vascular imaging, in many cases, it is important to be able toidentify the actual size of arteries from images from the X-rayradiography apparatus. This is because stenosis, a narrowing of theartery, is frequently treated by introducing a balloon into the arteryand then by dilating the balloon to the size of the healthy artery. Thesize of the healthy artery is determined by measuring the artery on eachside of the lesion due to the stenosis. This measurement is used toselect a balloon of suitable size to treat the stenosis. The images fromthe x-ray radiographs are projections. Consequently, a magnificationfactor is determined to indicate the actual size of the artery from itssize in the image. Several approaches have been proposed in order tocalculate the magnification factor. One approach currently used is tolocate a catheter (or any other object whose size is known) in the imageand to determine its size in the image. The actual size of the object isentered into the apparatus. Thus, the magnification factor for theobject is determined. Assuming that the distance to the projectioncenter is similar for the object and for the artery, the samemagnification factor is used to determine the size of the artery. Thisapproach has several disadvantages. First, it requires the user toprovide information which is not directly connected with the pathology,that is to say, the size of the instrument which is used as acalibration object. Should there be an error, a poor measurement ismade. Second, the applied algorithm assumes that the calibration objectand the artery to be measured are close to each other. This is almostnever the case. Furthermore, in certain situations, when the user wishesto employ a catheter as a calibration object, the catheter is notnecessarily visible on the image selected in order to measure theartery. This may lead to considerable errors of accuracy in determiningthe size of the artery, which is detrimental to the proper treatment ofthe stenosis.

BRIEF DESCRIPTION OF THE INVENTION

[0004] The present invention is to provide an embodiment of a method andan apparatus for determining the magnification factor. In an embodimentof the invention, a method of determining a magnification factor in aradiography apparatus of the type comprising means for providing anX-ray source and means for acquiring images, the source and the imageacquisition being mounted so as to rotate about at least one axis withrespect to an object to a support on which an object to be X-rayed isintended to be positioned, comprising acquiring at least two imagescorresponding to two different angular positions of the source and ofthe image acquisition with respect to the support; identifying on theseimages projections of at least one point of the X-rayed object; anddetermining the magnification factor of at least one of the images,first as a function of the angular displacement of the source and of theimage acquisition between the acquisitions of the images in questionand, secondly, as a function of the positions on these images of theidentified projections.

[0005] Further, an embodiment of the invention is directed to an X-rayradiography apparatus, comprising means for providing an X-ray sourceand means for acquiring images, the source and the image acquisitionbeing mounted so as to rotate about at least one axis with respect to asupport on which an object to be X-rayed is intended to be positioned,the apparatus comprising means for processing the images acquired byimplementing an embodiment of the method.

[0006] Also, in an embodiment of the invention, a method of acquiringvascular radiographic images by means of a radiography apparatus of thetype comprising means for providing an X-ray source and means foracquiring images, the source and the image acquisition being mounted soas to rotate about at least one axis with respect to a support on whichan object to be X-rayed is intended to be positioned, a method where amagnification factor is determined by implementing an embodiment of themethod.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The invention and embodiments thereof will become apparent fromthe following description and the appended drawings, in which:

[0008]FIG. 1 is a schematic view of a radiography device forimplementing an embodiment of a method for determining a magnificationfact or of a radiographic image; and

[0009]FIG. 2 is an outline diagram showing the taking of two images fromtwo different angles during an embodiment of the method.

DETAILED DESCRIPTION OF THE INVENTION

[0010] Referring to FIG. 1, an X-ray radiography apparatus 1 comprisesmeans for taking a radiography image 2 and means for emitting X-rays 3in the form of an X-ray source. The means for taking a radiographicimages 2 is, for example, a digital camera. The X-ray source 3 and thecamera 2 are attached to each end of an arm to form, in this example, asemicircle. The arm 7 is connected to a second arm 8 by, for example,sliding. The second arm 8 is itself connected, for example, by slidingand by rotating to a stand 9 of the radiography apparatus 1.

[0011] Arm 8 is capable of carrying out rotational movements 6 about itsown axis. Arm 7, capable of sliding with respect to the arm 8, makes arotational movement 5 with respect to the center of the semicircleforming arm 7.

[0012] In use, an object to be imaged, for example, the body of apatient, is positioned between the X-ray source 3 and the camera 2, sothat an artery 4 to be X-rayed is in the imaging field 10 of the device.Artery 4 is then at a distance z from the X-ray source 3. Byconstruction, the camera 2 is at a distance Z from this same X-raysource 3. The image of the artery 4 taken by the camera 2 is aprojection whose magnification factor f is equal to the ratio Z/z.

[0013] In order to be able to calculate the magnification factor f, theX-ray radiography apparatus 1 determines the distance z, given that thedistance Z is known because of construction of the apparatus. For thispurpose, with reference to FIG. 2, the radiography apparatus takes afirst image I₁ while the source 3 is in position X₁ in order to createan image at a first acquisition angle α₁ with respect to a reference R.The artery 4 is identified on the image I₁ by its projection 4′. Next,the X-ray radiography apparatus 1 takes a second image I₂ at a secondacquisition angle α₂ with respect to the reference R, the X-ray source 3then being in position X₂. Again, the artery 4 is identified on theimage I₂, by its projection 4″. Given the successive positions of theX-ray source 3 and X₁ and X₂, it is possible, by using triangulationcalculation methods, to determine, on the basis of projections 4′ and 4″of the artery 4, the spatial position of a point P of the artery 4 inthe imaging field 10 of the radiography apparatus 1.

[0014] The triangulation comprises the determination of the coordinatesof a point P belonging to the artery 4. For this purpose, the projectionP′ of the point P is identified on the image I₁. Given the coordinatesfor the position X₁ of the X-ray source 3, the equation of the straightline D₁ passing through X₁ and P′ is determined. Similarly, theprojection P″ of the P is identified on the image I₂. Given thecoordinates for the position X₂ of the X-ray source 3, the equation ofthe straight line D₂ passing through X₂ and P″ is determined. The pointP whose coordinates are sought is located in the middle of the segmentof the common perpendicular of the straight lines D₁ and D₂, connectingthe straight lines D₁ and D₂.

[0015] Given the spatial position of the point P of the artery 4, theradiography apparatus 1 can calculate the distance z separating theartery 4 from the X-ray source 3, for any one of the images taken.Hence, it thereby determines the magnification factor of this image inquestion and thereby deduces the actual size of the artery in question.

[0016] In order to displace the X-ray source from the position X₁ to theposition X₂, the arm 7 rotates about the artery 4, either in thedirection of rotation 6, or in the direction of rotation 5. Thedirection of rotation is chosen by the user according to the conditionsof use of the radiography apparatus 1. During this rotation, the X-rayradiography apparatus takes a series of successive images in a burst atan acquisition rate varying, for example, from 15 images per second to30 images per second. The series of images can be stored in a pluralityof memories (not shown) of the radiography apparatus 1.

[0017] In order to carry out triangulation to determine the spatialposition of the artery 4, the separation Δα between the angles α1 and α₂is, for example, between 15° and 45°. Preferably, the separation Δαisequal to 20°. In order to rotate through an angle Δα, the X-rayradiography apparatus 1 displaces the source 3 along one of theaforementioned directions of rotation 5, 6 at a rate of between, forexample, 30° per second and 40° per second. For example, the radiographyapparatus 1 is capable of taking a series of about 15 images for aseparation Δα of 20°, at a speed of 40° per second, and for an imageacquisition rate in a burst of 30 images per second. Over this series ofimages each one comprising a different projection of the artery 4, theradiography apparatus 1 will track the artery 4 by means of an imageprocessing method which is implemented by a processor (not shown) of theradiography apparatus 1 which has access to the plurality of memorieshaving stored the series of images. This image processing methodenabling tracking of this kind can be carried out, for example, in twoways: First, either the apparatus determines a region around the artery4 to be tracked and tracks this region throughout all the imagesconstituting the series of images by optimizing a similarity criterionsuch as the correlation, or second the apparatus segments the arteryover the first image and monitors this segmentation over the imagesconstituting the series. Such tracking methods are described in thefollowing publications: Zhaohua Ding & Morton H. Friedman“Quantification of 3-D coronary arterial motion using clinical biplanecineangiograms”, the International Journal of Cardiac Imaging, No. 16,pages 331 to 346, 2000; and Deriche Rachid and Faugeras Olivier,“Tracking line segment”, Image and vision computing, Volume 8, No. 4,pages 261 to 271, November 1990.

[0018] The X-ray radiography apparatus 1 implementing the method ofacquiring and monitoring an artery is designed to be used mainly duringsurgical procedures within an operating theatre. The user positions theradiography apparatus 1 around the object to image for example, apatient, so that the artery 4 which is desired to be studied is in theimage field 10 of the radiography apparatus 1. Since the camera 2 andthe X-ray source 3 are aligned at an angle α₁ with respect to thereference R, the user takes a first image which will indicate to theradiography apparatus the artery 4 which is to be studied. Next theradiography apparatus 1 rotates the camera 2 and the X-ray source 3,which are in alignment, to an angle α₂ with respect to the reference R.During this rotation through an angle Δα, the apparatus takes a seriesof images in a burst, as has been described above. Over this series ofimages, the apparatus tracks the designated artery 4, determines thespatial position of the artery 4 in the imaging field 10 of theradiography apparatus 1 by means of triangulation. The magnificationfactor f is determined in order to be able to provide the user with theactual size of the artery 4 which is to be studied, from the sizedetermined from the projection of the artery 4 on at least one of theacquired images. The apparatus will then be able to determine the exactshape of the artery in the form of variations of the cross section overa given segment, from the series of images.

[0019] Thus, it is no longer necessary for the user to enter the size ofa known object. The position of the artery is determined in the imagingfield of the radiography apparatus triangulation given the displacementangle and the position of the projections. Knowledge of this positionmakes it possible to determine the distance from the artery to the X-raysource. Knowing the distance form the image to the X-ray source of theradiography apparatus as a result of its construction, the magnificationfactor of this image can be accurately determined.

[0020] The embodiment of the apparatus and method has at least one ofthe following characteristics: at least two images on which anidentification is carried out for the purpose of determining amagnification factor are acquired for angular positions separated by anangle greater than 15°; at least two images on which an identificationis carried out for the purpose of determining a magnification factor areacquired for angular positions separated by an angle greater than 20°;during an image acquisition, a plurality of images is acquired between afirst and a second angular position; identification of the projectionsimplements automatic tracking of at least one point of the object fromone image to another, on the plurality of images acquired; the automatictracking implements monitoring by means of a similarity criterion of atleast one region of the object; the similarity criterion is acorrelation criterion; the automatic tracking implements monitoring ofat least one segment that is identified on the images.

[0021] Various modifications in structure and/or steps and/or functionmay be made by one skilled in the art without departing form the scopeand extent of the invention as recited in the claims.

What is claimed is:
 1. A method of determining a magnification factor ina radiography apparatus comprising means for providing an X-ray sourceand means for acquiring images, the source and the image acquisitionbeing mounted so as to rotate about at least one axis with respect to asupport on which an object to be X-rayed is intended to be positioned,comprising: (a) acquiring at least two images corresponding to twodifferent angular positions of the source and of the image acquisitionwith respect to the support; (b) identifying on these images projectionsof at least one point of the X-rayed object; an d (c) determining themagnification factor of at least one of the images, first, as a functionof the angular displacement of the source and of the image acquisitionbetween the acquisitions of the images in question and, secondly, as afunction of the positions on these images of the identified projections.2. The method ac cording to claim 1, wherein at least two images onwhich an identification is carried out for the purpose of determining amagnification factor are acquired for angular positions separated by anangle greater than 15°.
 3. The method according to claim 1, wherein atleast two images on which an identification is carried out for thepurpose of determining a magnification factor are acquired for angularpositions separated by an angle greater than 20°.
 4. The methodaccording to claim 2 wherein the angle is greater than 20°.
 5. Themethod according to claim 1 wherein during an acquisition step, aplurality of images is acquired between a first and a second angularposition.
 6. The method according to claim 2 wherein during anacquisition step, a plurality of images is acquired between a first anda second angular position.
 7. The method according to claim 3 whereinduring an acquisition step, a plurality of images is acquired between afirst and a second angular position.
 8. The method according to claim 1wherein during an acquisition step, a plurality of images is acquiredbetween a first and a second angular position.
 9. The method accordingto claim 2, wherein the identification of the projections implementsautomatic tracking of at least one point of the object from one image toanother, on the plurality of images acquired.
 10. The method accordingto claim 3, wherein the identification of the projections implementsautomatic tracking of at least one point of the object from one image toanother, on the plurality of images acquired.
 11. The method accordingto claim 4, wherein the identification of the projections implementsautomatic tracking of at least one point of the object form one image toanother, on the plurality of images acquired.
 12. The method accordingto claim 5, wherein the identification of the projections implementsautomatic tracking of at least one point of the object form one image toanother, on the plurality of images acquired.
 13. The method accordingto claim 6, wherein the automatic tracking implements monitoring bymeans of a similarity criterion of at least one region of the object.14. The method according to claim 13, wherein the similarity criterionis a correlation criterion.
 15. The method according to claim 5, whereinthe automatic tracking implements monitoring of at least one segmentthat is identified on the images.
 16. An X-ray radiography apparatuscomprising: (a) means for providing an X-ray source; (b) means foracquiring images; (c) the source and the means for acquiring imagesbeing mounted so as to rotate about at least one axis with respect to asupport on which an object to be X-rayed is intended to be positioned;(d) means for acquiring at least two images corresponding to twodifferent angular positions of the source and of the image acquisitionwith respect to the support; (e) means for identifying on these imagesprojections of at least one point of the X-rayed object; and (f) meansfor determining the magnification factor of at least one of the images,first, as a function of the angular displacement of the source and ofthe image acquisition between the acquisitions of the images in questionand, secondly, as a function of the positions on these images of theidentified projections.
 17. The X-ray radiography apparatus according toclaim 16 wherein at least two images on which an identification iscarried out for the purpose of determining a magnification factor areacquired for angular positions separated by an angle greater than 15°.18. The X-ray radiography apparatus according to claim 17 wherein atleast two images on which an identification is carried out for thepurpose of determining a magnification factor are acquired for angularpositions separated by an angle greater than 20°.
 19. The X-rayradiography apparatus according to claim 17 wherein the angle is greatthan 20°.
 20. A method of determining a magnification factor of anobject in a radiographic image comprising: providing an X-ray source;providing a means for acquiring images; determining a distance from themeans for acquiring images to the object; providing a first projectionof the object by taking a first image of the object while the source isin a just position in order to create the first image as a first anglewith respect to a reference; providing a second projection of the objectby taking a second image of the object while the source is a secondposition in order to create the second image as a second angle withrespect to the reference; calculating on the basis of the projections aspatial position of a point in the object; and calculating the distancefrom the source to the object based on the spatial position to determinethe magnification factor.
 21. The method of claim 20 wherein themagnification factor is determined: (a) first, as a function of theangular displacement of the source and the means for acquiring imagesbetween the acquisitions of the images; and (b) secondly, as a functionof the position on these images of the first and second projections. 22.The method of claim 20 wherein at least two images on which anidentification is carried out for the purpose of determining amagnification factor are acquired for angular positions separated by anangle greater than 15°.
 23. The method of claim 20 wherein at least twoimages on which an identification is carried out for the purpose ofdetermining a magnification factor are acquired for angular positionsseparated by an angle greater than 20°.
 24. The method of claim 20wherein the identification of the projections implements automatictracking of at least one point of the object from one image to another,on the plurality of images acquired.
 25. The method of claim 20 whereinthe automatic tracking implements monitoring by means of a similaritycriterion of at least one region of the object.
 26. The method of claim20 wherein the similarity criterion is a correlation criterion.
 27. Themethod of claim 20 wherein the automatic tracking implements monitoringof at least one segment that is identified on the images.
 28. The methodof claim 20 comprising taking a series of successive images in a burstas an acquisition rate varying from 15 images per second to 30 imagesper second.
 29. The method of claim 20 wherein the first and secondangles have an angular separation between 15° and 45°.
 30. The method ofclaim 29 wherein the angular separation is 20°.
 31. The method of claim20 wherein the X-ray source and the means for acquiring images rotateabout at least one axis relative to the reference as a rate of between30° per second and 90° per second.
 32. The method of clam 21 comprising:taking a series of 15 images for the angular separation of 20° as arational speed of 40° per second; and for an image acquisition rate in aburst of 30 images per second.
 33. A method for acquiring vascularradiographic images by means of a radiography device comprising an X-raysource and means for acquiring images placed facing the source, thesource and the means for acquiring images being mounted so as to rotateabout at least one axis with respect to a support on which an object tobe X-rayed is intended to be positioned, comprising determining amagnification factor by: (a) acquiring at least two images correspondingto two different angular positions of the source and of the imageacquisition with respect to the Support; (b) identifying on these imagesprojections of at least one point of the X-rayed object; and (c)determining the magnification factor of at least one of the images,first, as a function of the angular displacement of the source and ofthe image acquisition between the acquisitions of the images in questionand, secondly, as a function of the positions on these images of theidentified projections.